1. Field of the Invention
The invention relates to a method for reducing the level of carbon-containing particle emissions from diesel engines, in which the exhaust gas emitted by the engine flows through a filter, the particles contained in the exhaust gas are separated out at filter surfaces, and in which the particles which have been separated out are oxidized in order for the filter to be regenerated. In addition, the invention also relates to an associated system for carrying out the method, having a ceramic particulate filter and means for oxidizing the particles which are separated out at the ceramic filter.
2. Description of the Related Art
Recent studies have shown that particulates which reach the lungs are harmful to health and possibly even carcinogenic. However, in particular the direct injection diesel engines used for passenger automobiles, which are of interest for reasons of fuel economy, emit particles which reach the lungs.
One solution to the problem, which has long been proposed, could lie in regeneratable particle filters which, however, for regeneration at low exhaust temperatures require an additive, such as for example cerium, Na—Sr mixture or Fe—Sr mixture in the fuel, which acts as a catalyst for the oxidation of particulates (FR 2 771 449 A). Such catalysts act, for example, by first of all being oxidized themselves and then transferring the oxygen to the particulates.
In practical use, however, the oxides are only partially reduced by the particulates, so that there is a problem with catalyst ash blocking the filter in long-term operation. Additional problems arise with sulfur-containing fuels as a result of catalytically promoted sulfate formation. Furthermore, in urban traffic the problem whereby the exhaust-gas temperature is not sufficient for regeneration despite the use of a catalytic additive may arise.
On the other hand, purely thermal regeneration cannot be used, since this requires the engine to be briefly run at operating points with a greatly increased exhaust-gas temperature or requires electrical measures to be taken to heat the filter. For this purpose, EP 0 635 625 A1 discloses microwave heating of the filter ceramic, and EP 0 731 875 discloses an electrically heated oxidation catalytic converter for breaking down particulates. Each of these purely thermal measures entails a greatly increased mean fuel consumption. In addition, the combustion of the particulates initiated by these measures can cause the particulate filter to burn through locally, leading to it being destroyed.
By combining these two measures, it is possible to achieve an improvement, but this still does not solve the problem of the filter becoming blocked by catalyst ashes. Furthermore, under extreme circumstances (short-distance drives in urban traffic), the exhaust-gas temperatures may still remain so low that regeneration of the filter is nevertheless not possible.
To solve the problems relating to particulate emissions, plasma processes have been proposed or investigated a number of times in the past and can be classified as follows:    (a) Particles are electrically charged by treatment with a spray discharge, are electrostatically separated out and are oxidized on the substrate by plasma processes, if appropriate with the addition of a catalyst in the fuel or in the substrate (EP 0 332 609 B1, WO 91/03631 A1, U.S. Pat. No. 4,979,364 A; EP 0627 263 A1, DE 2 146228 A1).    (b) Particles are agglomerated by treatment with a spray discharge and are separated out by a cyclone (DE 34 24 196 A1 and EP 0 824 376 A1), where they are disposed of, for example thermally.    (c) Particles are separated out in a dielectric fixed bed of granules, in a fiber composite (felt) or in a porous material (ceramic foam or the like) as a filter. A non-thermal plasma is burnt in this porous structure, continuously regenerating the surfaces (WO 99/38603 A1).    (d) Plasma-induced regeneration of particulate filters can also be achieved if, in a non-thermal plasma, NO is oxidized to form NO2, which even at low temperatures is reduced again to form NO, with the particulates being oxidized. Given sufficient exhaust-gas temperatures, it is also possible to use an oxidation catalyst instead of the plasma (DE 198 26 831 A1 and EP 341 832 B1).    (e) Particles are separated out on structural electrodes of a DBD reactor, where DBD stands for dielectric barrier discharges) by inertia forces and are then oxidized by the action of non-thermal plasma (DE 100 07 130 A0).    (f) As they flow through a porous ceramic which acts as a filter, particles are retained and are oxidized by the action of a DBD plasma (DE 197 17 890 A1). According to one aspect of the invention, the DBD plasma is formed between a counterelectrode, which is provided with a barrier layer, and a gas-permeable electrode, which is connected to the filter ceramic, or a filter ceramic of sufficient electrical conductivity as an electrode.
The following comments should be noted in connection with these processes:    Re. a) The electrostatic separation of particles requires two plasma reactors—a first for electrically charging the particles proportionally to their mass, and a second for electrostatic separation and catalytic or plasma-induced oxidation. In a compact structure which is suitable for motor vehicles, this function cannot reliably be ensured. There is a risk of uncontrolled deposition of the particles at locations in the exhaust section at which their oxidation is not ensured. This can lead to sudden, uncontrolled release of large quantities of particles (re-entrainment).    Re. b) Even with electrostatic agglomeration, it is impossible to ensure that the particles are subsequently separated out in a controlled manner. This results in the same problems as those involved in electrostatic separation in (a).    Re. c) The separation of particulates in continuously plasma-regenerated porous structures has a good effect. However, there are problems with the long-term mechanical strength of the porous structure when used in vehicles (granules, fiber material) or with the dynamic pressure (ceramic foams).    Re. d) The continuous regeneration of particulate filters by an upstream plasma works in principle but requires the presence of sufficient quantities of NO in the exhaust gas and is disadvantageous in terms of energy (B. M. Penetrante et al.: Feasibility of Plasma Aftertreatment for Simultaneous Control of NOx and Particulates; SAE paper No. 1999-01-3637).    Re. e) Since the electrode structure is pervious to the exhaust gas, a low dynamic pressure does result and there is little likelihood of the exhaust-gas cleaning element becoming blocked by solid deposits, but since inertia forces drop as the mass falls, light particles substantially follow the gas flow and are therefore not separated out to a sufficient extent.    Re. f) With dielectric barrier discharges, a significant proportion of the power is converted in the volume. The remainder of the electric power can be converted on electrically insulating surfaces, such as the dielectric coating at what is known as the root of the discharge filament formed at atmospheric pressure. Therefore, in the case of an electrically conductive porous filter ceramic, a root of this type forms only on the dielectric of the gas-impermeable counterelectrode. Chemically active free radicals, such as the atomic oxygen O formed from atmospheric oxygen and the hydroxyl radical OH formed from water, are known to have a very short lifespan in exhaust gas, of less than 200 μs. Consequently, there is little likelihood of free radicals which are formed in the volume oxidizing particulates which have been separated out on the filter ceramic. This reduces the efficiency of the plasma regeneration. Moreover, the proposed electrode geometries greatly restrict the cross section of flow of the individual filter passages. To keep the dynamic pressure of the plasma-regenerated filter at a low level, it is necessary to increase the volume compared to filters without these electrode structures.